1. Field of the Invention
The present invention relates generally to devices for measuring the profile of the surface of a sample such as a semiconductor wafer, and more particularly, to a device for measuring changes in the relative height or depth of microscopic surface features of a sample, such as a semiconductor wafer, that allows crater depth measurements to be made while depth profile analyses are proceeding.
2. Description of Related Art
Measurements of elemental surface composition are critical in the manufacture of electronic and optoelectronic devices. These electronic devices include known semiconductor integrated circuit devices, such as data processing and memory devices. Optoelectronic devices, such as semiconductor lasers, comprise a fast growing area of modern technology and is key in many advances in the communications and information industries. Constant increases in the density of the devices situated on a semiconductor chip and reductions in device dimensions severely strain current production and characterization technologies.
During manufacture of these devices, constraints such as multiple surface layers, controlled dopants, engineered composition gradients, and unwanted impurities combine to determine the operation and yield of the manufactured device. Secondary ion mass spectrometry (SIMS), X-ray photoelectron spectrometry (XPS), and Auger electron spectrometry (AES) are essential methods used for surface analysis of trace elements and major components in these materials. Due to their sensitivity to the near-surface region of a material, these techniques allow measurement of material properties as a function of depth beneath the surface, an important procedure known as depth profiling.
In typical depth profiling, continuous or periodic ion beam sputtering forms a crater of microscopic dimensions in the surface of a sample, thus removing the surface material to expose progressively deeper material for further analysis. After the measurement is completed, samples are removed from the analytical instrument and sputtered crater depths are measured using a stylus profilometer or optical profilometer. Average known sputter rates are used to convert signal intensity versus the time that a measurement signal was received from the sample, to obtain data for comparing the composition of the material and the depth of the crater formed by the ion beam. Because different materials sputter at different rates, analysts use relative sputter rates measured in separate reference materials to adjust the average sputter rates calculated from crater depths. A disadvantage of these known methods is that shrinking device geometries, use of inhomogeneous materials, and increasing demands for ever tighter controls on dopant placement are maximizing the capabilities of these procedures.
In fabrication of high-speed electronic and optoelectronic devices, tight control of layer thickness, layer composition, and the precise positioning of dopant impurities relative to layer boundaries is critical. A disadvantage of the prior art, which uses external profilometers, for example to quantify the selenium (Se) and oxygen (O) concentrations in silicon containing multiple doped layers, is that it is necessary to make certain assumptions to apply sputter yield corrections in every layer. Thus, accurate depth measurements require pre-knowledge of the relative sputter rate for each layer of the sample. Calibration of the depth scale is based on a final crater depth measurement, and may not be sufficiently accurate for determining the time at which intermediate depths were reached.
A commercially available surface profiler sold under the trademark "MP2000", manufactured by Chapman Instruments, Inc., Rochester N.Y., comprises a stand-alone unit configured as a differential scanning interferometer that can be used for measuring crater depths after analysis of the sample is completed. The surface profiler incorporates a three-point phase shifting signal processing scheme for providing the device with a step height resolution of approximately 10 nm. However, a disadvantage of the disclosed surface profiler is that it cannot measure samples mounted inside of the vacuum chamber of a typical SIMS, XPS, or AES instrument. The sample must be removed and situated near the profilometer for measuring the surface of the sample. Another disadvantage of the disclosed surface profiler, and common to profilometers in the prior art, is that it does not provide real-time depth measurements of craters in the surface of the sample, as the craters are being formed.
Other known depth measuring devices include simple reflection monitors, which measure fringes in a reflected beam, and ellipsometers, which use polarizers to determine small phase shifts in the reflected light, each of which are used to measure film thicknesses. A disadvantage of each of these devices is that they are dependent on multiple film surfaces and are not suited for determining etch depths in a single surface.
Commercially available Michelson-type interferometers are used for position encoding on translation stages and for velocity measurements. These devices are used for measuring translations of the sample relative to a reference sample. However, a disadvantage of these devices is that they measure translations of the sample as a whole, and thus any motion of the sample relative to the external reference sample, such as caused by vibration, prevents the device from generating differential measurements that indicate the topography of the surface of the sample.
U.S. Pat. No. 5,017,012, to Merritt, Jr., et al., assigned to Chapman Instruments, Inc., Rochester N.Y., discloses a previewing profiler that includes apparatus to scan the surface of an object and to provide a display relating to the smoothness of the surface at a microscopic level. The system provides a polarized, collimated laser beam through a Nomarski-type prism and focuses the resulting beams on the surface to be scanned. A user operable rotatable mirror may be inserted between the laser and Nomarski prism. The mirror is designed to leak a small percentage of the laser light.
Another source of noncollimated polarized light, provided through a condensing lens, is provided to the rotatable mirror to be directed along the same path through the Nomarski prism and to be focused at a point above the surface being scanned, thereby providing a substantially larger illuminated area on the surface. The reflected light from both the laser beam and additional noncollimated light is focused on a charge coupled device (CCD) array and then displayed on a display. This permits the user to view the area to be profiled, including the profile line, prior to operating the profiler.
U.S. Pat. No. 4,719,120, to Green et al., is directed to a method for detection of oxygen in thin films. The method includes determining the presence, during deposition of a first thin film layer, of a substance which escapes when the layer is cooled and transferred from its deposition environment for analysis to determine the presence of the substance. The layer is first covered with a second layer of a material that captures the escaping substance. This second layer is then covered with a cap layer of a substance which seals the second layer against contamination, as from the atmosphere during transfer. The layered structure, with the escaped substance retained in the second layer, is then analyzed, as by sputter depth profiling and Auger electron spectroscopy, to determine the presence in the second layer of the escaped substance and thus determine the presence of this substance during deposition of the first layer.
Presently, there are a number of SIMS, XPS, and AES analytical instruments for surface analysis of samples and for determining the major components of the materials comprising the samples. However none of these known analytical instruments possess the capability of measuring crater depths. This may be due to the difficulty of performing crater depth measurements distally, inside an analytical vacuum chamber, with limited access to the sample and while the sample is vibrating, the analytical instrument is vibrating, or potentially both.
Thus, there exists a need for a device for measuring the depth of craters in the surface of a sample such as a semiconductor wafer and while depth profile analyses of the sample are proceeding.